Today we needed to find the IP of one of our routers which was not in use for some time. At first we could just reset the configuration and everything would be ok, but we didn't want to lose it.
Using nmap we were able to find its IP:
nmap -v -n -sP 10.2.0.0/16
With those parameters nmap will scan through al possible IPs in that network and report some information about it, which in our case was enough to determine what type of device it is and find the router.
As an example, the command output:
Nmap scan report for 10.2.3.1 Host is up (0.000078s latency). MAC Address: 48:5B:39:BB:53:BA (Asustek Computer)